The present invention relates to a frequency-division multiplex communication system for multiplex transmission, such as a CATV system.
In the prior art, a system as shown in FIG. 1 has been known for a frequency division multiplex communication system of the same general type to which the invention relates. In this known system, an output signal from a video signal source 1 is amplitude modulated by a video modulator 2. A synchronizing signal present in that output signal is frequency multiplied by n by a frequency multiplier 3. Further, a first audio signal 21 is FM modulated by a first-audio modulator 4-1, and a second audio signal 22 is FM modulated by a second audio modulator 4-2. The two modulated signals are mixed by a mixer 5. A phase detector 6, employing a PLL loop utilizing the output from the frequency multiplier 3 as a reference signal which has a frequency of n times the horizontal synchronizing frequency of the video signal, produces a difference frequency signal between the respective output signals from the audio modulators 4-1 and 4-2. In turn, the output signal from the phase detector 6 is used as the oscillator input to the second audio modulator 4-2. A composite television signal is formed from the output signals from the video modulator 2, first audio modulator 4-1 and second audio modulator 4-2 and sent out by a signal transmitting unit 7 as a multiplex signal.
In the multiplex signal 23 obtained in this way, the second audio carrier signal is disposed at a frequency higher than the first audio carrier frequency specifically, by a frequency of n.multidot.f.sub.H (f.sub.H is the horizontal synchronizing frequency). The level of the second audio carrier signal is set to a level lower than that of the first audio carrier level by 20 dB and more.
The TV signal including the foregoing multiplex signal 23 is applied to a TV receiver. Specifically, the TV signal is applied to the RF input of a tuner 8. The output of the tuner 8 is coupled to a mixer 9 and thence to an IF amplifier circuit 11. A local oscillator signal is supplied to the second input to the mixer 9 by an oscillator 10. The output of IF amplifier circuit 11 is AM detected by a detector 12, and the video component of detector output is applied through a video amplifier 13 to drive a cathode-ray tube 25. Further, an intercarrier output signal included in the detected output is FM detected and amplified by an audio detector/amplifier 14, and the resultant demodulated signal is reproduced by a loudspeaker 24.
A portion of the output signal from the IF amplifier circuit 11 is applied to a second IF amplifier circuit 15, the passband of which is set to emphasize the second modulated signal. The output of IF amplifier circuit 15 is inter-carrier detected by a detector 16. The thus-obtained signal is processed by a bandpass filter 17 with which the first audio signal and other unnecessary signal components are eliminated. The resulting signal is FM demodulated by an FM detector 18 and input to a reproducing unit 20 through an amplifier 19.
In the case of the conventional frequency-division multiplex communication system utilizing an additional carrier signal as described above, the following problems are present:
(1) In order to reduce interference between the second audio signal and the video and/or the first audio signal, it is necessary to lower the level of the second audio relative to the video carrier level by 26 dB or more and to suppress the same below the level of the first audio signal. Thus, the bandpass filter 17 is required to have considerably sharp skirts for adequate separation of the modulated second audio signal.
(2) For easy separation of the modulated second audio signal, it is necessary to provide a large frequency difference between the first audio carrier and the second audio carrier. However, if this frequency difference is made excessively large, interference with the video signal results, particularly with the upper (higher frequency) side because it becomes necessary to push the carrier of the second audio signal to a lower frequency.
(3) In order to reduce "buzz", which can interfere with the second audio signal prior to demodulation, it is necessary to include a circuit, such as the second IF amplifier circuit 15, to emphasize the level of the modulated second audio signal, thereby to reduce the level difference relative to the video signal.
(4) In order to reduce the effects of "buzz", a direct receiving arrangement employing a split-carrier system has been known. However, an improvement of the selectivity thereof, as would be necessary to give fully acceptable results, gives rise to another troublesome problem, namely, the stability of the received frequency is required to be very high.
The present invention was achieved in view of the drawbacks described above in the prior art. Thus, an object of the invention is to provide a frequency-division multiplex communication system in which the second audio signal is spaced in frequency not far from the first audio carrier, has a level which gives rise to no interference with the first audio signal, does not require a receiver having a remarkably high selectivity, and is not subjected to "buzz" interference or interference with the first audio signal.